Proc. Aust. Soc. Anim. Prod. (1972) 9: 181 AGE OF ONSET OF PUBERTY IN MERINO EWES IN SEMI-ARID TROPICAL QUEENSLAND R. M. MURRAY* Summary TWO groups, each of 25 ewes were run with harnessed vasectomized rams from 9 months of age (spring born group) and 3 months of age (autumn born group) respectively. Weekly records were made for 24 months of liveweight and occurrence of oestrus. Puberty occurred at an average weight of 33.7 kg and 33.2 kg and an average age of 580 days and 537 days, for spring and autumn born ewes respectively. Most of the spring and autumn born ewes reached puberty during the same period, indicating that both weight and season of birth may influence the age of onset of puberty. I. INTRODUCTION The general practice in north-west Queensland is to mate maiden ewes at 30 months of age. Greater lifetime productivity would be gained if the age of maiden ewes at first joining was reduced to 18 months. However this would be dependent on the ewes having reached puberty by this age. Puberty in ewes has been studie.d by many workers (Roux 1936; Hammond 1944; Kelley 1946; Hafez 1951, 1952; Ch ang and Raeside 1957; Watson and Gamble 1961; Allen and Lamming 1961), and great variation of age onset has been observed. The age at which the first oestrus is shown is affected by season of birth (Watson and Gamble 1961). Hammond (1944) working with Suffolk ewes in England, found that although there was neither a fixed time of the year nor a fixed age at which oestrus first occurred, animals which were 180 days old by the middle of the normal breeding season exhibited oestrus during this season, whereas puberty was delayed until the following season in younger animals. Hafez (195 1) and Allen and Lamming (1961) consider that the age of puberty is influenced by growth rate as well as time of birth. It may be that environmental conditions retard sexual development in some tropical areas and rule out the joining maiden ewes at 18 months of age. This experiment was designed to yield information on this subject. II. MATERIALS AND METHODS Two groups each of 25 Merino ewes were studied. Group 1 ewes were born during an 8 week period in September, October, 1966 (spring group) and weaned in January 1967. The ewes in Group 2 were born during an 8 week period in March, April 1967 (autumn group) and weaned in July the same year. +: Department of Primary Industries, Richmond, Queensland, 4822. -i- Present address: Department of Biochemistry and Nutrition, University of New England, Armidale, New South Wales, 235 1.
The ewes were selected at random in July 1967 from the lambs of the autumn joined and spring joined flocks maintained at Toorak Research Station, Julia Creek, North Queensland. They were then run with sire-sine harnessed vasectomized rams for 24 months. Eighty-one mature ewes were run with the 50 young ewes in order to maintain a continuous check on the efficiency of the rams in detecting ewes in oestrus. The rams were added at a rate of 3 per cent of the ewes, and were exchanged for fresh vasectomized rams every 8 weeks. The animals grazed a 250 ha paddock. Weekly records were made of liveweight and occurrence of oestrus. At the first detected o-estrus the ewes were laparotomized and examined for evidence of ovulation. The presence of an active corpus luteum was taken as evidence of the ewe having reached puberty. The experiment was concluded after 24 months. III. RESULTS Growth of the young ewes All animals increased in weight except during two periods, November, 1967 to mid-january, 1968 and December 1968 to mid-march 1969 (Figure 1). At 12 months of age the spring born ewes had an average liveweight of 30.0 kg and the autumn born ewes averaged 24.5 kg. By 24 months of age the spring born group averaged 43.5 kg and the autumn born group weighed 37.0 kg. Eficiency of rams in detecting ewes in oestrus Information on the interval within which the mature ewes were marked by the rams following their introduction into the experimental groups is summarized in Table 1. The oestrus activity of these ewes during the period April 9 to June 4, TABLE 1 Eficiency of rams in detecting ewes in oestrus, as assessed by the result with mature ewes running with the young ewes 1968 was quite low, as 54.3 per cent failed to show oestrus, or mated only once. Nevertheless, only 3.7 per cent of the ewes had mating intervals greater than 21 days. It can therefore be concluded that the level of activity in the rams was uniformly high throughout the experiment.
0C First occurrence of oestrus The first occurrence of oestrus in the individual animals of each group is graphed in relation to the age of the animals and the season of the year in Figure 1. Data were not available for the individual birth dates, and the ages of the young ewes are taken from the mean of the distribution of lambing. The mean ages of the ewes at first oestrus were 580 days and 537 days for the spring and autumn born ewes respectively. The number of autumn born ewes reaching puberty before 18 months of age was significantly greater than that for the spring born ewes (X ( 1 ) = 9.707 : P<O.Ol). There was a significant difference (P<O.Ol) between the means for the time from introduction of the vasectomised rams to puberty, for the spring (289 days) and autumn born (415 days) ewes. The liveweight of the animals when oestrus was first detected varied from 22.7 to 46.3 kg. The mean liveweights at first oestrus for the two groups of ewes were not significantly different (spring born 33.7 kg and autumn born 33.2 kg). Although at no time during the experiment was there any significant difference between the liveweights of the pre-puberal and post-puberal ewes within the spring or autumn born groups, there was a highly significant difference between the liveweights of the spring and autumn groups themselves. There was a highly significant increase in the Iiveweight of ewes reaching puberty as the age at first oestrus increased. IV. DISCUSSION It is possible that some of the spring born ewes may have reached puberty before first contact with the rams at 9 months of age. However this is not considered likely as none of the young ewes were cycling when the rams were introduced, despite the fact that all members of the groups were increasing in weight. There was a large variation in the age at which puberty occurred, varying from 320 to 690 days in animals born in the spring, and from 424 to 802 days in the autumn born animals. However, the majority of ewes, irrespective of season of birth, exhibited oestrus for the first time during the four months May to August, 1968, when 18 spring born ewes then 20-25 months old, and 17 autumn born ewes then 13-16 months old, reached puberty. Although 2/3 of the ewes from both groups reached puberty within the same 4 month period, the difference between the corresponding means (580 and 537 days : Figure 1) was not significant statistically. As the spring born ewes were 6 months older than the autumn born ewes, it would indicate that season of birth contributed to age at onset of puberty, and that the 1 l/2 month difference in the mean age of the two groups at first oestrus (Figure 1) may be a real and biologically significant difference. There is substantial evidence that variation in day length exerts a controlling influence on the breeding season of the ewe (Yeates 1949, 1956; Hart 1956; Hafez 1952; Clegg, Cole and Ganong 1964). The small change which occurs in day length in the tropics may be insufficient to cause a photoperiodic effect in Merino ewes. However, puberty could be expected to be more sensitive than oestrus in adults. Also the photoperiod would be distorted in the area under study. The first longest day in November could well be followed by stimulation of puberty in early
January, followed by inhibition as day length again approached maximum with further stimulation as day length shortened after the last longest day in January. Although theoretical, this does offer a possible explanation of the peculiar peak, then gap, then peak, in incidence of first oestrus observed in the spring born group. However, Radford (1966) points out that no one hypothesis based solely on photoperiodism serves to explain all the observed facts on seasonal breeding. The age at which ewes reach puberty appears to be greater in the semi-arid tropics than in the temperate areas (Watson and Gamble 1961). This is probably a function of the poorer nutrition, and therefore slower growth rate of the ewes in north-west Queensland. The district practice of delaying joining of the maiden ewes until they- have reached 2 /2 years of age may not be necessary, as the autumn born ewes reached puberty at an average age of 18 months (537 days), and the spring born ewes at 19 months of age (580 days). This means that at least half the maiden ewes would have mated if joined at 11% years of age. In years when conditions allow rapid growth of the young ewes, and they reach liveweights in excess of 34 kg early in their lives, it could be expected that the majority of maiden ewes would mate at 11% years of age. In these circumstances, delaying mating until 2% years seems unjustified. V. ACKNOWLEDGMENT Thanks are due to Miss P. Pepper for assistance with the statistical analyses. VI. REFERENCES Allen, D. M. and Lamming G. E. (1961). Journal of Agricultural Science, Cambridge, 57: 87. Ch ang, T. S., and Raeside, J. E. (1957). Proceedings of the New Zealand Society of Animal Production, 17: 80. Clegg, M. T., Cole, H. H., and Ganong, W. F. (1964). Proceedings of the Conference of Oestrous CycZe Control, Domestic Animal Miscellaneous Publications. United States Department of Agriczzlture, No. 1005: 96. Hafez, E. S. E. (1951). Nature, London, 168: 1046. Hafez, E. S. E. (1952). Journal of Agricultural Science, Cambridge, 42: 189. Hammond, J. Jnr. (1944). Journal of Agricrcltural Science, Cambridge, 34: 97. Hart, D. S. (1956). Proceedings of the New Zealand Society of Animal Prodcction, 16: 101. Kelley, R. B. (1946). Council for Scientific and Industrial Research, Australian Bulletin No. 205. Radford, H. M. (1966). Proceedings of the Australian Society of Animal Production, 6: 19. ROUX, L. 1,. (1936). Onderstepport Journal of Veterinary Science, 6: 571. Watson, R. H. and Gamble, L. C. (1961). Australian Journal of Agricultural Research, 12: 124. Yeates, N. T. M. (1949). Journal of Agricultural Science, Cambridge, 39: 1. Yeates, N. T. M. (1956). Australian Journal of Agricultural Research, 7: 440. 185